Document U.S. Pat. No. 7,890,080 B2 discloses such a receiver that is used for radio frequency identification (RFID) devices like RFID readers to communicate with active or passive transponders. In a typical application a passive transponder or tag stores product identification of a product to which it is attached and the reader is used to obtain this product information. The reader is powered and generates a magnetic field from its antenna. When the reader and the tag are within close proximity of each other, the reader generated magnetic field is induced into the antenna of the tag and used to power the tag. The tag also has a transceiver to receive the signal from the reader and to transmit a response back to the reader.
There are standards like ISO/IEC18000-3 or ISO/IEC 14.443 Type A and B or ISO15.693 or ECMA-340 13.56 MHz Near Field Communication (NFC) or company standards like Felica from company Sony that define protocols and types of modulation used to transmit information between the tag and the reader. Some or all of these standards define that the reader transmits data to the tags by changing the magnitude of its transmitted power. Tags receive the transmitted signal and process the received data. The activated tag then replies by transmitting data to the reader. A typical technique is to use load modulation, in which the tag varies the load impedance of its coil by changing its resonance frequency and its quality factor. This action causes a voltage variation at the reader antenna. The receiver of the reader disclosed in FIG. 3 of document U.S. Pat. No. 7,890,080 B2 and shown in FIG. 1 of this patent application processes such load modulated analog input signals to output digital data detected in the input signal.
FIG. 1 shows receiver 1 that comprises a differential amplifier 2 to amplify the load modulated analog input signal from antenna 3. An in-phase mixer 4 mixes the input signal with an in-phase carrier frequency and provides an in-phase component I of the down-converted input signal and a quadrature-phase mixer 5 mixes the input signal with a quadrature-phase carrier frequency and provides a quadrature-phase component Q of the down-converted input signal. Amplifiers 6 and 7 amplify the in-phase component I and the quadrature-phase component Q of the down-converted input signal and DC block filter 8 and 9 remove the DC component of the in-phase component I and the quadrature-phase component Q, which in-phase component I and the quadrature-phase component Q are then digitized by analog-digital converters 10 and 11. The digitized components are then filtered by digital filters 12 and 13 and matched filters 14 and 15 and joined together in a digital decoder 16.
Drawback for this receiver disclosed in document U.S. Pat. No. 7,890,080 B2 is that the matched filtering is done after digital filtering and the characteristics of the down-converted sub-carrier are not exploited thus making it less robust against noise and interferers.